1. Field of the Invention
The invention relates to microlithographic projection exposure apparatuses as used for manufacturing highly-integrated electrical circuits and other microstructured components. In particular, the invention relates to projection exposure apparatuses configured for immersion operation. The invention further provides measuring devices for determining the imaging properties of projection lenses.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are usually manufactured by applying a plurality of structured layers to a suitable substrate, which may be, for example, a silicon wafer. To structure the layers, they are first covered with a photoresist that is sensitive to light of a given wavelength range, e.g. light in the deep ultraviolet (DUV) spectral range. The coated wafer is then exposed in a projection exposure apparatus. A pattern composed of structures located on a mask is imaged on the photoresist by means of a projection lens. Because the imaging scale is generally less than 1:1, such projection lenses are frequently referred to as reduction lenses.
After the photoresist has been developed the wafer is subjected to an etching or deposition process whereby the uppermost layer is structured according to the pattern on the mask. The remaining photoresist is then removed from the remaining parts of the layer. This process is repeated until all the layers have been applied to the wafer.
One of the primary design objectives in the development of projection exposure apparatuses is to be able to lithographically define structures of increasingly small dimensions. Small structures lead to high integration densities, which generally have a favourable effect on the efficiency of microstructured components manufactured using such apparatuses.
The size of the definable structures depends, above all, on the resolution of the projection lens used. Because the resolution of projection lenses improves as the wavelength of the projection light becomes shorter, one approach to decrease the resolution is to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 193 nm and 157 nm, i.e. in the deep ultraviolet (DUV) spectral range.
Another approach to decrease the resolution is based on the concept of introducing an immersion liquid having a high refractive index into the space located between a last lens of the projection lens on the image side and the photoresist or another photosensitive layer to be exposed. Projection lenses, which are designed for immersion operation and are therefore also referred to as immersion lenses, can attain numerical apertures of greater than 1, e.g. 1.3 or 1.4. However, immersion not only makes possible high numerical apertures and therefore an improved resolution, but also has a favorable effect on depth of focus. The greater the depth of focus, the less high are the demands for precise positioning of the wafer in the image plane of the projection lens.
A projection exposure apparatus designed for immersion operation is known from U.S. Pat. No. 4,346,164 A. To accommodate a wafer, this known projection exposure apparatus has an upwardly open container with an upper edge that is located higher than the lower boundary face of the last lens of the projection lens on the image side. Inlet and outlet pipes for an immersion liquid open into the container. These pipes are connected to a pump, a temperature-stabilizing device and a filter for cleaning the immersion liquid. During operation of the projection exposure apparatus, the immersion liquid is circulated in a loop. An immersion space located between the lower boundary face of the last lens of the projection lens on the image side and the semiconductor slice to be exposed remains filled the immersion liquid.
A projection exposure apparatus having an immersion arrangement is also known from WO 99/49504. In this projection exposure apparatus the supply and discharge pipes for the immersion liquid open directly onto the lower boundary face of the last lens of the projection lens on the image side. The use, in particular, of a plurality of such supply and discharge pipes, which may be arranged, for example, in a ring around the last lens on the image side, makes it possible to dispense with a surrounding container. This is because immersion liquid is sucked away as it runs off laterally and is fed back in such a way that the immersion space between the last lens on the image side and the photosensitive surface always remains filled with immersion liquid.
A difficulty with the immersion operation of projection exposure apparatuses is to keep the optical characteristics of the immersion liquid constant, at least where the liquid is exposed to the projection light. Special attention must be paid to the absorption and the refractive index of the immersion liquid. Local fluctuations in the absorption, as can be produced, for example, by impurities, lead to undesired intensity fluctuations in the image plane. As a result, line width fluctuations may occur even if the imaging is otherwise free of substantial aberrations.
Local fluctuations in the refractive index of the immersion liquid have an especially detrimental effect, since such fluctuations directly impair the imaging characteristics of the projection exposure apparatus. If the refractive index of the immersion liquid is inhomogeneous within the volume of the immersion liquid exposed to the projection light, this causes distortions of the wave fronts passing through the immersion space. For example, points in the object plane of the projection lens may no longer be imaged sharply on the image plane.
The refractive index of liquids is dependent on their density. Because liquids are virtually incompressible, their density is practically independent of static pressure and depends almost exclusively on the temperature of the liquids. For this reason, the immersion liquid inside the immersion space that is exposed to the projection light can have a homogeneous refractive index only if the temperature of the immersion liquid is constant therein. Moreover, temperature fluctuations within the immersion liquid not only cause fluctuations of refractive index, but can also cause adjacent optical elements, in particular the last optical element of the projection lens on the image side, to be heated unevenly and therefore to be deformed in a manner that can hardly be corrected.
The causes that give rise to inhomogeneities of the temperature in the immersion space are diverse. A major cause for heating the immersion liquid is the absorption of projection light by the immersion liquid. Even if only a small percentage of the projection light is absorbed by the immersion liquid, this causes a comparatively high heat input because of the short-wave and therefore energy-rich projection light. An effect which leads to cooling of the immersion liquid is the evaporation of immersion liquid at the boundary surface to a surrounding gas. In addition, the temperature of the immersion liquid is influenced by heat transitions from and to surrounding solid bodies. These bodies may be, for example, a heated last lens of the projection lens, its housing or the wafer to be exposed.
To homogenize the temperature, it has been proposed hitherto to circulate the immersion liquid in a circuit and to establish a desired reference temperature by means of a temperature-stabilizing device. However, the homogenization of temperature distribution that can be achieved in this way is frequently not sufficient. Relatively high flow velocities, which may lead to disturbing vibrations, are usually required. In addition, high flow velocities promote the formation of gas bubbles which can also adversely affect the imaging properties.
Moreover, similar difficulties also arise in measuring devices with which the imaging characteristics of such projection lenses can be determined. If immersion lenses having numerical apertures of greater than 1 are to be measured, it is also necessary to introduce an immersion liquid into an immersion space located between the last optical element of the projection lens on the image side and a test optics component of the measuring device. Because of the extremely high demands on the measuring accuracy of such devices, inhomogeneities of refractive index within the immersion liquid cannot be tolerated.